Cr23C6 and Cr7C3 carbides in heat resistant steel

Cr23C6 and Cr7C3 carbides in heat resistant steel

Table 1: Chemical composition of the steel samples (wt.%).

Figure 1: As-cast microstructure of steel 1. Scale bars: 100 µm.

Figure 2: As-cast microstructure of steel 2. Scale bars: 100 µm.

Table 2: Carbides observed in the steel samples.

Figure 3: Microstructures of steels 1 and 2 after heat treatment at 1023 K for 50 h, of the as-cast samples; (a) steel 1, (b) steel 2. Scale bars: 50, 20 µm.

Figure 4: Microstructures of steels 1 and 2 after heat treatment at 1023 K for 50 h, of the samples homogenized at 1323 K. (a) Steel 1, (b) steel 2. Scale bars: 20 µm.

Carbide name: Cr23C6, Cr7C3
Record No.: 1031
Carbide formula: Cr23C6, Cr7C3
Carbide type: M23C6, M7C3
Carbide composition in weight %: No data
Image type: LM, SEM
Steel name: Heat resistant steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Heat-resistant steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: The chemical composition of the castings analyzed in this study, as determined by means of emission spectrometry (SPECTROLAB), is given in Table 1, where the samples were labeled as steels 1 and 2. The figures match the usual composition range stated for HC-type steel (UNS J92605, A297, A608); slight differences in composition can be observed for corresponding elements in both samples. The Sigma-phase precipitation treatment for steels 1 and 2 (1023 K for 50 h) was performed on the as-cast microstructure and also after homogenization annealing at 1323 K for 6 h. The microstructural characterization of both alloys was carried out by means of quantitative metallography (ZEISS AXIOTECH, OLYMPUS BX60 and OLYMPUS VANOX) and scanning electron microscopy (JEOL JSM 840). The specimens were etched with Emanuel reagent (30 g K3FeCN6, 30 g KOH, 60 ml H2O) at 323 K for 25 s. The component phases were quantitatively determined by image analysis. On the other side, the carbides were identified by means of Xray diffraction (SIEMENS D5000, monochromatized Cu Ka radiation, k ¼ 0:15406 nm, 40 kV, 30 mA) after extraction by electrochemical dissolution at 1,5 V (5 g C2H2O4, 200 ml HCl, 1000 H2O).
Note: The influence of solidification microstructure on r-phase precipitation in two refractory stainless steels (26Cr–3Ni) with different microstructures is analyzed. Depending upon cooling rates the existence of Cr7C3 strongly increases the r-phase precipitation rate, whereas Sigma precipitation is delayed when this carbide is not present in the alloy.

The microstructure of the as-cast samples is shown in Figs. 1 and 2, where the bright etching constituent is austenite while ferrite is depicted by the dark etching areas. In Fig. 1(a) two types of primary carbides can be observed, one of them being distributed as small islands inside the austenite grains and the other one precipitated as an eutectoid component along the austenite–ferrite interface. On the other side, only one type of carbide was found in steel 2, as disclosed in Fig. 2(a). The X-ray identification of the carbides is detailed in Table 2; here Cr23C6 and Cr7C3 were detected in steel 1 in the weight ratio of 49/51 while only Cr23C6 was contained in steel 2.
The total amount of extracted carbide was approximately 7 wt.%. Figs. 1(b) and 2(b) show the size of the solidification dendrites, which are very much smaller in steel 2 as compared with steel 1, which suggests that steel 2 solidified at a faster rate. The microstructure of the ascast samples after 50 h of annealing at 1023 K are shown in Fig. 3, where it can be observed that r-phase nucleates along the austenite–ferrite interface and grows into the ferrite grains. The relative amount of r-phase was about 20 wt.% in steel 1 but only a few % in steel 2. On the other hand, Cr23C6 was the only type of carbide contained in both steels after homogenization of the ascast samples at 1323 K for 6 h. In fact, according to Table 2 the Cr7C3 had completely disappeared. Samples treated at 1023 K for 50 h after homogenization at 1323 K are shown in Fig. 4, where Sigma-phase is not yet visible.
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